272 research outputs found
Computational screening study towards redox-active metal-organic frameworks
The metal-organic framework (MOF) MFU-4l containing Co(II) centers and Cl-
ligands has recently shown promising redox activity. Aiming for further
improved MOF catalysts for oxidation processes employing molecular oxygen we
present a density-functional theory (DFT) based computational screening
approach to identify promising metal center and ligand combinations within the
MFU-4l structural family. Using the O2 binding energy as a descriptor for the
redox property, we show that relative energetic trends in this descriptor can
reliably be obtained at the hybrid functional DFT level and using small cluster
(scorpionate-type complex) models. Within this efficient computational protocol
we screen a range of metal center / ligand combinations and identify several
candidate systems that offer more exothermic O2 binding than the original
Co/Cl-based MFU-4l framework.Comment: 9 pages, 4 figure
Magneto-optical probing of weak disorder in a two-dimensional hole gas
In two-beam magneto-photoluminescence spectra of a two-dimensional valence
hole gas we identify the three-level energy spectrum of a free positive trion
with a field-induced singlet-triplet transition. The recombination spectrum of
acceptor-bound trions is also detected, including a cyclotron replica
corresponding to the hole shake-up process. The emergence of a shake-up peak at
low temperature is shown to be a sensitive probe of the presence of a small
number of impurities inside the high-mobility quantum well, and its relative
position is directly related to the hole cyclotron mass.Comment: 4 pages, 5 figure
Magneto-capacitance probing of the many-particle states in InAs dots
We use frequency-dependent capacitance-voltage spectroscopy to measure the
tunneling probability into self-assembled InAs quantum dots. Using an in-plane
magnetic field of variable strength and orientation, we are able to obtain
information on the quasi-particle wave functions in momentum space for 1 to 6
electrons per dot. For the lowest two energy states, we find a good agreement
with Gaussian functions for a harmonic potential. The high energy orbitals
exhibit signatures of anisotropic confinement and correlation effects.Comment: 3 pages, 3 figure
Noise thermometry in narrow 2D electron gas heat baths connected to a quasi-1D interferometer
Thermal voltage noise measurements are performed in order to determine the
electron temperature in nanopatterned channels of a GaAs/AlGaAs heterostructure
at bath temperatures of 4.2 and 1.4 K. Two narrow two-dimensional (2D) heating
channels, close to the transition to the one-dimensional (1D) regime, are
connected by a quasi-1D quantum interferometer. Under dc current heating of the
electrons in one heating channel, we perform cross-correlated noise
measurements locally in the directly heated channel and nonlocally in the other
channel, which is indirectly heated by hot electron diffusion across the
quasi-1D connection. We observe the same functional dependence of the thermal
noise on the heating current. The temperature dependence of the electron
energy-loss rate is reduced compared to wider 2D systems. In the quantum
interferometer, we show the decoherence due to the diffusion of hot electrons
from the heating channel into the quasi-1D system, which causes a thermal
gradient.Comment: 6 pages, 5 figure
Single-hole transistor in p-type GaAs/AlGaAs heterostructures
A single-hole transistor is patterned in a p-type, C-doped GaAs/AlGaAs
heterostructure by AFM oxidation lithography. Clear Coulomb blockade resonances
have been observed at T=300 mK. A charging energy of ~ 1.5 meV is extracted
from Coulomb diamond measurements, in agreement with the lithographic
dimensions of the dot. The absence of excited states in Coulomb diamond
measurements, as well as the temperature dependence of Coulomb peak heights
indicate that the dot is in the multi-level transport regime. Fluctuations in
peak spacings larger than the estimated mean single-particle level spacing are
observed.Comment: 4 pages, 5 figure
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